1. Field of the Invention
The present invention relates to an integrated circuit, non-contact IC card, a reader/writer, a wireless communications method, and a computer program.
2. Description of the Related Art
Non-contact IC (Integrated Circuit) cards are widely in use in which an IC chip is embedded in a card to execute wireless communication with a reader/writer (R/W) terminal in a noncontact manner. Especially, non-contact IC cards are widely in use in ticket check processing at ticket gates of public transportation, payment processing at checkout counters of retail stores, and clock-in/clock-out management at offices, for example.
A non-contact IC card contains no power supply. Therefore, the non-contact IC card uses, as its power supply, a voltage that is generated when the IC card is held over a reader/writer terminal through an antenna coil that receives a magnetic field generated by the reader/writer terminal. The voltage generated as an induced voltage is supplied to each component of the IC card as the power. The non-contact IC card and the reader/writer terminal are communicating each other by use of the radio wave of a predetermined frequency. The non-contact IC card and the reader/writer terminal each have a frequency resonance circuit that resonates at that predetermined frequency. For example, in the ISO14443 standard that is the communications standard of the non-contact IC card, the resonance frequency is set to 13.56 MHz.
The response from the non-contact IC card or the IC chip embedded in a portable information processing terminal is supported by “load modulation”. In load modulation, the load inside the IC chip is turned on/off to change the impedance of the IC chip relative to the reader/writer terminal. The reader/writer terminal detects the change in the power consumption of the IC chip as a voltage change of the carrier (the card or the portable terminal). Therefore, the reader/writer terminal can recognize a response from the IC chip by the occurrence of a voltage change.
Recently, a method is known in which an IC chip is incorporated in not only a non-contact IC card but also a portable information processing terminal, such as a mobile phone, and this terminal is held over a reader/writer terminal, thereby executing wireless communication with the reader/writer terminal. Holding such a portable terminal over a reader/writer terminal connected to a ticket check system of public transportation or a register machine at a retail store allows the execution of ticket check processing or payment processing by use of a portable information processing terminal. Especially, at ticket gates or checkout counters, it is required to execute communication at high speeds so as not to put the users of portable information processing terminals into a waiting line. For this purpose, a technology for stabilizing the response from the IC chip is disclosed.
The above-mentioned technology is disclosed in Japanese Patent Laid-open No. 2001-222696 (Patent document 1). Patent document 1 discloses a method in which the capacities of capacitors in a resonance circuit in a non-contact IC card is changed to shift the resonance frequency of the resonance circuit, thereby improving the response from the IC card.